A Publication of the Delta Stewardship Council

Quagga Mussels - Our Next Alien Invader?

Department of Fish and Game K-9 Unit
Inspecting for Quagga Mussels
(Photo courtesy of California Department of Fish and Game)

After first appearing in the Great Lakes in the late 1980s, zebra and quagga mussels spread swiftly across the central and eastern United States. They were widespread in the Mississippi drainage by 1994 and have continued to multiply. In 2007 quagga mussels crossed the Rocky Mountains and were found in Lake Mead on the Arizona/Nevada border.

Within months, they had spread to Riverside, San Bernardino, and San Diego counties via aqueducts carrying water to southern California from the Colorado River. An apparently separate invasion of zebra mussels was discovered in San Justo Reservoir in San Benito County just 15.5 miles southwest of San Luis Reservoir and the Delta watershed. Although their expansion in California appears to have slowed, some think it is only a matter of time before these invasive species find their way into the Delta.

This map shows the current distribution of zebra and quagga mussels in California:

“Dreissenid (zebra and quaqqa) mussels have rapidly spread across the United States, leaving ecosystem destruction and impaired water supply features in their wake,” says Bruce Herbold, an estuarine ecology consultant and fish biologist recently retired from the U.S. Environmental Protection Agency. “Their impacts to open systems in the Central Valley is likely to be profound, as it has been everywhere else that they have invaded.”

Efforts to control the spread of Dreissenid mussels through education, inspection, and mandatory boat cleaning appear to have slowed their advance in California. The California Department of Fish and Wildlife has mussel management strategy goals in place to:

Prevent further introductions into the State

Contain mussels within currently infested waters

Eradicate mussels from infested waters if feasible

Information about the State program to control zebra and quagga mussels can be found here.

“Everything that is being done at present is focused on keeping them out; as it has been in every area they have previously invaded,” Herbold says. “It seems unlikely that they will be kept out, but every effort should be made to delay their arrival so that we can plan our response and anticipate their impacts.”

Already impacted by at least two other invasive filter feeding bivalves, the Asian clams Corbicula fluminea and Potamocorbula amurensis, the Delta can ill afford another species that can further reduce its aquatic food supply. Quagga and zebra mussel invasions in other water bodies have caused major shifts in the aquatic food web by removing huge amounts of phytoplankton, the microscopic plants that other species depend upon. Populations of zooplankton and fish that depend on them for food are also reduced.

Quagga and Zebra Mussels Clogging a Water Pipe
(Photo courtesy of California Department of Water Resources)

These introduced mussels can also become abundant enough to clog water pipes increasing the maintenance costs of water agencies. This invasive species destroys fisheries, damages boats, and cripples water systems.

Quagga and Zebra Mussels
(Photos courtesy of California Department of Fish and Game)

Like their marine relatives such as the common blue mussel (Mytilus edulis) and California mussel (Mytilus californianus), these two freshwater bivalves, the zebra mussel (Dreissena polymorpha) and the quagga mussel (Dreissena rostriformis bugensis), are usually attached to solid objects but can also survive on sandy or muddy bottoms. They are relatively small, usually less than an inch long, but can be present in huge numbers. Densities as high as 35,000 per square meter have been reported.

Also, like their marine relatives, they can produce huge numbers of microscopic free swimming larvae that are carried along with the current helping them to invade new territory downstream. Native to eastern Europe and western Asia, these Dreissenid mussels likely came to North America in the ballast water of ships. Since then, they have been able to invade new water bodies attached to the hulls of boats or as larvae in water contained in boat hulls or engines.